Network interface units (NIU's) are located at the point where telephone company owned transmission facilities meet subscriber owned facilities. These units respond to test signals from a test device usually from the telephone central office side of the unit and identify whether a reported fault is located on the telephone company side or the subscribe side of the unit.
These NIU's may be located in a device as shown in U.S. Pat. No. 4,488,008 issued Dec. 11, 1984 for a single subscriber. Alternatively, many such NIU's may be located in a device as shown in U.S. Pat. No. 4,213,013 issued July 15, 1980 for many subscribers. Yet another device for many subscribers comprises devices which clip on to a cluster of four terminals of a connector block such as the 66 type connector manufactured and sold by AT&T-Technologies, Inc. A problem with a clip-on device, however, occurs when the terminals are bent, in which case the device is rendered inoperable. When the terminals are to be tested, existing test probes cannot be used without removing the device, in which case service is interrupted.